samv7/sam_serial.c: reset all DMA pointers on DMA setup

This small fix ensures all DMA pointers are correctly reseted during
DMA setup (when the driver is opened). Without this there could be rare
occurrence of driver pointer to incorrect (invalidate) DMA buffer and thus
saving incorrect characters to upper layer.

Signed-off-by: Michal Lenc <michallenc@seznam.cz>

arch/arm/src/samv7/sam_serial.c

@@ -1228,6 +1228,8 @@ static int sam_dma_setup(struct uart_dev_s *dev)
        */
 
       priv->rxdmanext = 0;
+      priv->nextcache = 0;
+      priv->buf_idx = 0;
       priv->rxenable = true;
       priv->odd = false;